Superconducting quantum interference device (SQUID) magnetometers are known for measurement of magnetic fields in both the DC and radio frequency (RF) regimes. See, for example, J. Zimmerman et al., Appl. Phys. Lett., 51, p. 617 (1987).
An RF SQUID consists of a single non-hysteretic Josephson device, such as a Josephson junction, a weak link, or a point contact junction, wherein the Josephson device interrupts a superconducting current. An RF SQUID acts as a two-terminal element with a non-linear current-voltage relationship. The exact current-voltage relationship is determined by the strength of a magnetic flux intercepted by the SQUID. This magnetic flux dependence can be used as a basis for a variety of measurement devices, such as volt meters, ammeters, gradiometers, microwave phase shifters, filters, mixers, microwave detectors and magnetometers.
For example, voltage fluctuations in a current-biased, unshielded SQUID provide an indication of changes in a local applied magnetic field. Typically, an LC-resonant circuit, such as an oscillator tank circuit, is placed near the SQUID so that an RF signal may be introduced. The magnetic field generated by the LC-resonant resonant circuit inductively couples to the SQUID, influencing the voltage across it. In particular, the inductive coupling detunes the LC-resonant circuit from its natural resonating frequency. The extent of coupling between the LC-resonant circuit and the SQUID is frequency dependent. Consequently, the loading of the LC-resonant circuit due to coupling is also frequency dependent. Additionally, the loading of the resonant circuit depends on the internal differential inductance of the SQUID device, which varies periodically with an applied magnetic flux. The result is a system with an output response amplitude that varies non-linearly with its input driving amplitude, and in accordance with an applied magnetic flux. Thus, reflected microwave power varies in accordance with a magnetic flux applied to the SQUID device due to non-linear behavior of the LC-resonant circuit, thereby allowing detection and measurement of the local magnetic flux. When a quasistatic magnetic flux that is intercepted by the SQUID is changed, the voltage amplitude measured across the resonant circuit oscillates. The voltage across the resonant circuit is then detected and amplified.
One advantage of a higher frequency magnetometer device lies in the broader bandwidth that would be available for the detection of dynamic magnetic field signals. However, known RF SQUIDs can inductively couple only to signals with frequencies of less than 430 MHz, and cannot couple to high frequency RF signals that exceed 1000 MHz, while maintaining high sensitivity.